Ultrasonic Non-destructive Characterization And Evaluation Of The Elastic Modulus Of Thin Metal Sheet And Spheroidized Structure In Low Carbon

Thin elastic layer is widely applied to the fields of microelectronic,automobile and aerospace industry because of its excellent performances(such as wear resistance,corrosion resistance and oxidation resistance etc).Elastic modulus is not only the necessary parameter in the design of thin element but also the foundation of measurement and evaluation of the following mechanical properties,such as yield strength,fracture toughness and residual stress etc.Therefore,more and more people pay attention to the research on elastic modulus of thin elastic layer.As ultrasonic testing technique was successfully applied in characterizing the mechanical properties of solid medium,researchers are full of confidence to characterize elastic modulus of thin elastic layer by it.A novel ultrasonic longitudinal reflection wave frequency-domain method suitable for measuring the thickness of thin elastic layer is presented.The acoustic pressure transmission coefficient of thin elastic layer was derived.The elastic modulus of the thin layer was inversed based on the least squares error sense,the inversing results with various frequencies were compared with those obtained by velocity method.Main factors of estimate deviation were analyzed at the same time.The sensitivity function of the acoustic pressure transmission coefficient to elastic modulus of the layer and its importance in the transfer of errors were discussed.The results show that,comparing with the idealized value,the relative errors of Young's modulus and Poisson's ratio are both 2.9%for aluminum flake,while the relative errors are 6.25%,8.2%for sheet steel under the nominal frequency of 2.5MHz.When the frequency changes,the relative errors are 5.1%,4.4%for aluminum flake and 9.3%,5.3%for the sheet steel.It is observed in the experiment that this method can characterize elastic modulus of thin elastic layer effectively,and it can be a supplement for the measurement of elastic modulus of lamellar materials.Error analysis showed that the accuracy of this method is mainly dependent on the sensitivity function of acoustic pressure transmission coefficient to elastic modulus of the layer and the measurement error.20 steel is widely used to the aspects of boiler steel,steam turbine and pipeline due to its outstanding comprehensive mechanical properties.At room temperature and normal atmosphere,the stable microstructure of this steel is lamellar pearlite and ferrite,but when some factors such as high temperature,stress etc act on it,a series of microstructure evolution, such as pearlite spheroidization and graphitization etc will take place.Such evaluation will cause the decrease of toughness and strength of materials.Simultaneously,it can increase the risk of equipments failure.Therefore,it is important to implement field monitoring and life assessment for heat resistance parts in order to ensure the extended-used safety running at high temperature.There is abundant information related to structures of materials in the echo signal, therefore,the information parameters change with the variation of structures.Based on the previous theory,ultrasonic velocity technique was proposed to evaluate spheroidizate microstructure evolution of 20 steel.At first,the longitudinal and transverse wave velocities in 20 steel were measured through the method of the ultrasonic impulse echoes.Then elastic properties indexes were calculated based on the equation of Christoffel.and evaluation of microstructure morphologies was analyzed combining with the equilibrium diagram of the samples.Furthermore,the damage degree of the samples was characterized by damage variable to accomplish ultrasonic nondestructive evaluation of material microstructure.It is observed in the experiment that Young modulus,shear modulus and bulk modulus of 20 steel decreases as aging time increases.However,Poisson's ratio changes slightly.The relative variation is 6.3%,5.98%,7.56%,and 1.5%respectively.When the aging time exceeds 2300h. the damage variation of elastic modulus will be up to 0.07;the decrease of elastic modulus in aging process is related to morphology and distribution of cementite.